The effects of the compatibilization on the toughening of polypropylene (PP) by melt blending with styrene/ethylene-butylene/styrene triblock copolymer (SEBS) in a twin-screw extruder were investigated. The compatibilizers used were an SEBS functionalized with maleic anhydride, a PP functionalized with acrylic acid, and a bifunctional compound, p-phenylenediamine (PPD). The effects of the compatibilization were evaluated through the mechanical properties and by the determination of the phase morphology of the blends by scanning electron microscopy. Reactive compatibilized blends show up to a 30-fold increase in impact strength compared to neat PP, which was likely to have been due to the reaction of the bifunctional compound (PPD) with the acid acrylic and maleic anhydride groups, which rendered both morphological and mechanical stability to these blends. The addition of the PPD to the blends significantly changed their phase morphologies, leading to larger dispersed particles' average diameters, probably due to the morphological stabilization at the initial processing steps during extrusion, with the occurrence of the chemical reactions. (C) 2003 Wiley Periodicals, Inc.